# std::ranges::views::all, std::ranges::views::all_t

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
inline constexpr /* unspecified */ all = /* unspecified */; | (1) | (since C++20)
template< ranges::viewable_range R > using all_t = decltype(views::all(std::declval<R>())); | (2) | (since C++20)

1) A RangeAdaptorObject (also a RangeAdaptorClosureObject) that returns a `view` that includes all elements of its `range` argument.
Given an expression e of type `R`, the expression views::all(e) is expression-equivalent to:

- Implicitly converting e to a std::decay_t<R> prvalue, if std::decay_t<R> models `view`.
- Otherwise, std::ranges::ref_view{e} if that expression is well-formed.
- Otherwise, std::ranges::owning_view{e}.

2) Calculates the suitable `view` type of a `viewable_range` type.

### Example

Run this code

    #include <iostream>
    #include <ranges>
    #include <type_traits>
    #include <vector>

    int main()
    {
        std::vector<int> v{0, 1, 2, 3, 4, 5};
        for (int n : std::views::all(v) | std::views::take(2))
            std::cout << n << ' ';
        std::cout << '\n';

        static_assert(std::is_same<
            decltype(std::views::single(42)),
            std::ranges::single_view<int>
            >{});

        static_assert(std::is_same<
            decltype(std::views::all(v)),
            std::ranges::ref_view<std::vector<int, std::allocator<int>>>
            >{});

        int a[]{1, 2, 3, 4};
        static_assert(std::is_same<
            decltype(std::views::all(a)),
            std::ranges::ref_view<int[4]>
            >{});

        static_assert(std::is_same<
            decltype(std::ranges::subrange{std::begin(a) + 1, std::end(a) - 1}),
            std::ranges::subrange<int*, int*, std::ranges::subrange_kind(1)>
            >{});
    }

Output:

    0 1

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3724 | C++20 | `views::all` might result in hard error for some move-only views | it gets well-constrained
P2415R2 | C++20 | `views::all` returned a `subrange` for a non-`view` rvalue `range` | returns an `owning_view` for it

### See also

ranges::empty_viewviews::empty (C++20) | an empty `view` with no elements (class template) (variable template)
ranges::single_viewviews::single (C++20) | a `view` that contains a single element of a specified value (class template) (customization point object)
ranges::owning_view (C++20) | a `view` with unique ownership of some `range` (class template)

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